In electrophotography, a latent image is created on the surface of an imaging member such as a photoconducting material by first uniformly charging the surface and then selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner.
Typically, a dual layer electrophotographic photoconductor comprises a substrate such as a metal ground plane member on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated. The charge transport layer contains a charge transport material which comprises a hole transport material or an electron transport material. For simplicity, the following discussions herein are directed to use of a charge transport layer which comprises a hole transport material as the charge transport compound. One skilled in the art will appreciate that if the charge transport layer contains an electron transport material rather than a hole transport material, the charge placed on a photoconductor surface will be opposite that described herein.
Generally, when the charge transport layer containing a hole transport material is formed on the charge generation layer, a negative charge is typically placed on the photoconductor surface. Conversely, when the charge generation layer is formed on the charge transport layer, a positive charge is typically placed on the photoconductor surface. Conventionally, the charge generation layer comprises a polymeric binder containing a charge generation compound or molecule while the charge transport layer comprises a polymeric binder containing the charge transport compound or molecule. The charge generation compounds within the CGL are sensitive to image-forming radiation and photogenerate electron-hole pairs within the CGL as a result of absorbing such radiation. The CTL is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of a negatively charged photoconductor. Photoconductors of this type are disclosed in the Adley et al U.S. Pat. No. 5,130,215 and the Balthis et al U.S. Pat. No. 5,545,499.
Generally, by increasing the content of the charge transport compound in the charge transport layer, faster imaging speeds and reduced residual voltages may be obtained. However, when the amount of the charge transport compound in the charge transport layer is increased above about 40 to 50 weight percent, based on the weight of the charge transport layer, the mechanical properties of the photoconductor often begin to suffer and increased wear rates and reduced mechanical strength result. Several references have disclosed the use of certain amounts of specific charge transport compounds or the use of a charge transport polymer in charge generation layers, for example in the Champ et al U.S. Pat. No. 4,490,452, the Kato et al U.S. Pat. No. 4,882,253 and the Umeda et al U.S. Pat. No. 5,677,094. However, as photoconductors of improved sensitivity and durability and improved performance over extended photoconductor life are continually desired, particularly at lower cost, there is a continuing need for the development of new materials and methods to meet these demands.